A typical halogen lamp headlight system includes a reflector and a lamp capsule placed with its filament at or near the focal point of the reflector. The reflector commonly includes a parabolic or ellipsoidal rear portion and flat wall sections between the rear portion and the front of the assembly. The front is usually covered with a transparent lens. Typically, the lens, the reflector, or a combination of both is designed to direct the light from the light source into a specified pattern.
It is critical, particularly in low beam applications, to control the light output of the headlight so that stray light, or glare, does not shine in the eyes of the drivers of oncoming cars. In order to control the light output of the headlight, it is necessary to control the light which is emitted from the lamp capsule so that reflections from non-parabolic or non-ellipsoidal portions of the reflector are minimized. Low beam headlights are typically designed such that light which is emitted at 10.degree. to 90.degree. up from the axis of the lamp capsule is blocked from exiting the headlight, because this light forms what is known as veiling glare. Light from the lamp capsule is also prevented from impinging on the lens directly, because this also contributes to veiling glare. In general, for low beam headlights, efforts are taken to ensure that the light which is emitted from the lamp capsule, but which is not directed toward the parabolic or ellipsoidal portion of the reflector, is prevented from striking the reflector surface or from exiting the headlight system. The light which is reflected from the parabolic or ellipsoidal portion of the reflector is more controlled and is less likely to cause glare in the headlight system.
Several prior art methods have been used to control the light emitted from lamp capsules in vehicle headlight systems. One common method is to cover the portion of the lamp capsule which faces the lens with an opaque, low reflectivity material that prevents transmission of light from this portion of the lamp capsule. A headlight wherein the central portion of the lamp capsule is coated with a film of mercury to render it opaque is disclosed in U.S. Pat. No. 1,671,900 issued May 29, 1928 to Irwin. An incandescent lamp having a top portion of the lamp coated with a non-reflective, opaque coating of manganese dioxide and aluminum phosphate is disclosed in U.S. Pat. No. 4,288,713 issued Sep. 8, 1981 to Marlor. A halogen lamp having an opaque coating of sodium silicate and manganese dioxide for masking portions of the light output is disclosed in U.S. Pat. No. 4,391,847 issued Jul. 5, 1983 to Brown.
Another method of controlling light in headlight systems involves placing a light shield inside the headlight reflector. The light shield is commonly a non-reflective black metal which can withstand the high temperature generated by the lamp capsule. The light shield effectively blocks the unwanted light emitted by the lamp capsule. Still another method involves mounting a refractory metal shield inside the lamp capsule near the filament. These methods of controlling the light emitted by the lamp capsule result in inefficiencies in the headlight system, because they prevent a large percentage of the light emitted by the lamp capsule from exiting from the headlight system.
Automobile headlights wherein a portion of the lamp capsule has a reflective coating are disclosed in the prior art. U.S. Pat. No. 1,974,982, issued Sep. 25, 1934 to Christmas, discloses an automobile headlight wherein a portion of the lamp is silver plated to control the light output from the headlight system. The Christmas patent states that light reflected by the silvered area to the reflective panels is fanned out because this reflected light comes from beyond the focus of the reflective panels. An automobile headlight wherein a portion of the lamp is rendered opaque by silvering is disclosed in U.S. Pat. No. 1,578,079 issued Mar. 23, 1926 to Davis. The silvered portion reflects light emitted by the filament onto the main reflector. A disadvantage of this configuration is that light rays reflected by the reflective coating onto the main reflector may potentially cause glare, because they do not originate at the focal point of the main reflector and, thus, are outside the desired light pattern.
Tungsten halogen lamps having specular reflective coatings which function as reflectors for directing light in a desired direction are disclosed in U.S. Pat. No. 3,983,513 issued Sep. 28, 1976 to DeCaro. An infrared heater wherein a filament is mounted in a reflector having an infrared reflective coating is disclosed in U.S. Pat. No. 3,445,662 issued May 20, 1969 to Langley. U.S. Pat. No. 4,864,181, issued Sep. 5, 1989 to Huijbers et al, discloses an incandescent lamp having a truncated conical filament with a zigzag configuration and a mirror coating with a central opening on the lamp envelope. The disclosed lamp, when mounted in a parabolic reflector, is used as a traffic signal lamp. U.S. Pat. No. 4,758,761, issued Jul. 19, 1988 to Ingeveld et al, discloses an electric lamp wherein a portion of the lamp envelope is coated with an aluminum layer, and a boundary of the aluminum layer has a transparent aluminum oxide layer.
Interference filters have been used with tungsten halogen lamps to increase efficiency. The interference filter is formed as a multi-layer coating on the outer surface of a spherical or elliptical lamp envelope. The interference filter passes visible light and reflects infrared radiation back to the filament, causing an increase in filament temperature and operating efficiency. An example of a tungsten halogen lamp having an infrared reflective coating is disclosed in U.S. Pat. No. 4,942,331, issued Jul. 17, 1990 to Bergman et al.